CS 6543 Computer Networks
Spring 2004 - Midterm
April 19, 2004
Name:………………………Score: ……./30
- (5 points) In this question, you will explain/discuss some concepts briefly.
- (2pt) Suppose a router has built up the routing table shown as below. The router can forward packets to routers R2, R3, or R4.
Dest. SubnetNext hop
128.96.8.0/21R2
128.96.12.0/22R3
128.96.14.0/23R4
What will be the next node for a packet whose destination address is128.96.13.10
Next hop: (Don’t forget to show how you get your answer):
- (1pt) What is the difference between link-state routing and distance-vector routing?
Name:……….
- (1pt) Recall the operation of BGP, and specifically explain how BGP can detect and avoid loops in paths?
- (1pt) What is the goal of QoS routing? Giving a scenario, explain the need for it?
Name:……………….
- (5 points) Consider the below Local Area Network (LAN).
Suppose node A has an IP packet to send to node C. List all the steps that are performed until the packet reaches to network layer of node C. Don’t forget to mention what B does!
Assume that node A knows the IP address of node C and has a complete routing table. However, physical addresses are NOT known by any NIC.
Steps:
1. Node A looks up the routing table to identify the next node for 1.1.1.3 and determines that the destination node is in the same network
2. …..
Name:…………..
- (5 points) To provide reliable data transfer between directly and indirectly connected nodes, we use the same mechanisms (e.g., ACK, timeout, sliding win etc). But in the case of indirectly connected nodes, we face different challenges.
List these differences/challenges and briefly explain how they are dealt with by TCP.
Name…………….
- (5 points) Consider the below network. Users X and Y are sending data to X’ and Y’, respectively, using TCP. Assume that this TCP uses a simple congestion-control algorithm as in homework problem 17 in ch 6, i.e., it uses linear increase and multiplicative decrease but no slow start and it works in units of fixed-size packets. In case of multiplicative decrease, use w= w/2.
- Suppose at time t=0 the window size of X is 2 while that of Y is 6.
- RTT is 1 sec for both connections X—X’ and Y—Y’, all the packets in the current window are sent and ACKed in one RTT and the timers are set to RTT.
- If router A gets more than 10 packets/sec it will drop at least one packet from both X and Y.
- (3pt) Give a detailed sketch of congestion window sizes at X and Y until at least t=19sec.
0 / 1 / 2 / 3 / 4 / 5 / 6 / 7 / 8 / 9 / 10 / 11 / 12 / 13 / 14 / 15 / 16 / 17 / 18 / 19
!!!!! Using these figures answer parts (b) and (c) in the next page !!!!
Name:…………….
- (1pt) What is the throughput for X and Y until they start equally sharing the link A—B
- (1pt) What is the throughput for X or Y after they start equally sharing the link A—B.
Name:……………………………..
- (5 points) Consider the below weighted fair queuing (WFQ). Assume A, B, C generate same size packets.
If all nodes had always packets to send, then the output sequence would be
AAABBC AAABBC…
- (2 pt) Assume that all nodes generates 2 packets/sec at the beginning of each second. Starting from A, what would be the output sequence? Justify your answer.
- (3pt) Assume that A generates 1 packet/ sec,
B generates 3 packet/sec, and
C generates 2 packet/sec at the beginning of each second.
Starting from A, what would be the output sequence? Justify your answer.
Name:……………………………
- (5 points) Consider the below leaky bucket that controls the sending rate of S1. Ignore all processing and propagation delays.
Suppose S1 generates 2 packets at the beginning of every second and puts them in to queue if space is available.
- (3pt) When the total number of packets generated by S1 will be more than the total number of packets that can be transferred by the leaky bucket?
Total number generated by S1
Total number that can be sent
Time / 0 / 1 / 2 / 3 / 4 / 5 / 6 / 7 / 8 / 9 / 10 / 11 / 12 / 13 / 14 / 15
- (2pt) At what time, the first packet will be dropped due to buffer overflow?
1